The present invention relates generally to the field of electric light dimmers and controls, and more particularly to a system for controlling and eliminating undesirable characteristics associated with conventional electric light dimmers, such as what is known as snap on hysteresis effects.
Conventional low cost light dimmers of the type a consumer might purchase from his or her local hardware or mass merchandizing store presently exhibit an undesirable characteristic which will herein be referred to as the snap on hysteresis effect. This effect manifests itself in the operation of a dimmer by causing the lamp to which the dimmer is connected (such as an incandescent electric light bulb) to turn on at an initial brightness level somewhat and often significantly higher than the minimum brightness level achievable. This effect is both unexpected and undesirable.
Typically, dimmer designs today incorporate semiconductor devices that perform the dimming function, and comprise an AC switch and a trigger control circuit to control the AC switch. Conventional trigger circuits employ diacs connected to the gate of a triac which acts as the AC switch to turn the triac on and turn off. A phase shift circuit used with the diac determines where in the half cycle of the AC voltage supply wave the triac fires, thereby determining the duration of time current flows through the lamp which, in turn, determines its brightness. A typical phase control circuit of the type presently being sold by Leviton Manufacturing Co., Inc. of Little Neck, N.Y., for example, utilizes a trigger that exhibits symmetrical electrical characteristics during both positive and negative half cycles of the AC voltage. Due to characteristics of the triac, however, the phase control circuit exhibits asymmetrical electrical characteristics when controlling the gate of a triac. The end result is that the triac triggers earlier in the half cycle than it would otherwise have and at a higher than minimum brightness level. A user, when confronted with the higher than desired brightness level, will back off the control (turn the control knob, for example, in the dimming direction) to achieve brightness levels closer to the minimum achievable level.
In addition to the undesirable higher initial brightness level of the lamp, another drawback of the snap on effect is that if power is interrupted and the brightness was backed off after initial turn on, and thereafter the power was restored, the light might not come on at all. Furthermore, if the phase control circuit utilizes a series of stepped resistances instead of a potentiometer, it is not possible with such conventional devices to reach relatively lower brightness level.
Prior art attempts to solve this snap on hysteresis problem have included utilizing two circuits to control the firing of the triac, rather than a single circuit. The first such circuit controls the timing (i.e. brightness) of firing, while the second circuit controls the charge dumping of the capacitor into the gate of the triac. Drawbacks with this prior art approach include an increase in cost and and added complexity of the overall circuitry.
Another attempted solution to the snap on hysteresis problem is to use an asymmetrical trigger in the phase control circuit. This can be done using a number of discrete components or using AC triggers having asymmetrical electrical characteristics. The resulting devices, however, are not believed to be commercially available and having to add a number of components to a phase control circuit design to achieve the effect of an AC trigger increases its cost and complexity.
The result has been a long felt need for a solution to this problem that is simple and effective, and yet costs very little in terms of component cost and circuit complexity.